Clearly identify 32-bit and 64-bit times in serialization and proptests

[teor2345]

Scheduling

This risk is acceptable for the stable release, but we need to fix it before we support lightwalletd.

Is your feature request related to a problem? Please describe.

Zcash has two serialized time fields: 32-bit seconds, and 64-bit seconds. Zebra also keeps internal 64-bit times with additional 32-bit nanosecond precision.

In general, it's hard to identify, convert, serialize and proptest these different time types correctly.

This is a follow-up to #1849.

Describe the solution you'd like

Make wrapper types for serialized times:

• DateTime32 (Add a DateTime32 type for 32-bit serialized times #2210)
• DateTime64

Tasks for each wrapper type:

• make arbitrary impls
  • the arbitrary impls should replace the existing datetime_full and datetime_u32 proptest strategies
  • we can use datetime_full for DateTime64 by removing nanoseconds
• make serialization impls

Replace chrono::DateTime<Utc> with the appropriate types:

• 32-bit serialized: DateTime32 Use DateTime32 for block times and lock times #2211
• 64-bit serialized: DateTime64
• internal monotonic: std::time::Instant
• Remove unused dependencies on chrono

Describe alternatives you've considered

It would be nice if Zcash just used 64-bit times throughout the protocol.

[dconnolly]

It would be nice if Zcash just used 64-bit times throughout the protocol.

💕

[teor2345]

I did DateTime32 in #2210 so we could do #2178

[teor2345]

This is a useful validation improvement, but it's not needed right now.